Researching this, I also found about TSWindowClipper which allows you to seamlessly integrates remote apps on your client by integrating a DLL inside the MSTSC software using the official virtual channels.

Back to rdesktop: I really wish the documentation was better, but it contains some very interesting source code.

rdesktop is an open source client for Windows Remote Desktop Services, capable of natively speaking Remote Desktop Protocol (RDP) in order to present the user’s Windows desktop. rdesktop is known to work with Windows versions such as NT 4 Terminal Server, 2000, XP, 2003, 2003 R2, Vista, 2008, 7, and 2008 R2.

rdesktop currently runs on most UNIX based platforms with the X Window System, and other ports should be fairly straightforward.

rdesktop is released under the GNU Public Licence (GPL), version 3. Please send feedback, bug reports and patches to the appropriate mailing list. Patches can also be submitted to the SF patch tracker.

The latest stable version of rdesktop is 1.7.1 (edit: this was at the time of writing, for the current latest, check here). This versions solves major issues with 64bit version and smartcard support among a few minor fixes.